Add a way to decode the event back to the original scitt signed state…

…ment payload (#17)

* Add a way to decode the event back

to the original scitt signed statement payload

---------

Co-authored-by: jgough <[email protected]>

scitt/decode_event.py

@@ -0,0 +1,91 @@
+""" Module for decoding the event """
+
+import argparse
+
+import json
+import base64
+
+from pprint import pprint
+
+from pycose.messages import Sign1Message
+
+
+def open_event_json(event_json_file: str) -> bytes:
+    """
+    opens the event json
+    """
+    with open(event_json_file, "rb") as file:
+        event_json = file.read()
+        return event_json
+
+
+def get_base64_statement(event_json: bytes) -> str:
+    """
+    gets the base64 encoded signed statement from
+    the datatrails event
+    """
+
+    event = json.loads(event_json)
+
+    base64_signed_statement = event["event_attributes"]["signed_statement"]
+
+    return base64_signed_statement
+
+
+def decode_base64_statement(base64_statement: str) -> bytes:
+    """
+    decodes the base64 encoded signed statement
+    into a cbor cose sign1 statement
+    """
+    signed_statement = base64.b64decode(base64_statement)
+    return signed_statement
+
+
+def decode_statement(receipt: bytes):
+    """
+    decodes the signed statement
+    """
+
+    # decode the cbor encoded cose sign1 message
+    message = Sign1Message.decode(receipt)
+
+    return message
+
+
+def main():
+    """Verifies a counter signed receipt signature"""
+
+    parser = argparse.ArgumentParser(
+        description="Verify a counter signed receipt signature."
+    )
+
+    # signing key file
+    parser.add_argument(
+        "--event-json-file",
+        type=str,
+        help="filepath to the stored event, in json format.",
+    )
+
+    args = parser.parse_args()
+
+    event_json = open_event_json(args.event_json_file)
+
+    base64_signed_statement = get_base64_statement(event_json)
+    print(f"\nbase64 encoded signed statement: \n\n{base64_signed_statement}")
+
+    signed_statement = decode_base64_statement(base64_signed_statement)
+    print(f"\ncbor encoded signed statement: \n\n{signed_statement}")
+
+    decoded_statement = decode_statement(signed_statement)
+
+    print("\ncbor decoded cose sign1 statement:\n")
+    print("protected headers:")
+    pprint(decoded_statement.phdr)
+    print("\nunprotected headers: ")
+    pprint(decoded_statement.uhdr)
+    print("\npayload: ", decoded_statement.payload)
+    print("payload hex: ", decoded_statement.payload.hex())
+
+
+if __name__ == "__main__":
+    main()